In July 2008 Dan Kaminsky disclosed a discovery of a significant number of problems with “DNS Cache Poisoning”. The DNS vulnerability stems from shortcomings in the way servers try to ensure IP address information comes from bona fide sources rather than those controlled by miscreants. To prevent tampering, DNS queries include a random transaction number. The response is only considered valid only if it contains the same number.
What Kaminsky discovered is that this safeguard can be easily circumvented because there were only 65,536 possible transaction IDs. By flooding a DNS server with multiple requests for a domain name with slight variations, for example, 1.google.com, 2.google.com, 3.google.com and so on, and then sending many bogus responses with many different transaction ids, an attacker can vastly improve the chances of the DNS server accepting one of his bogus responses.
The United States Computer Emergency Readiness Team (US-CERT) issued a Vulnerability Note VU#800113:
Multiple DNS implementations vulnerable to cache poisoning
Overview—Deficiencies in the DNS protocol and common DNS implementations facilitate DNS cache poisoning attacks.
I. Description—The Domain Name System (DNS) is responsible for translating host names to IP addresses (and vice versa) and is critical for the normal operation of internet-connected systems. DNS cache poisoning (sometimes referred to as cache pollution) is an attack technique that allows an attacker to introduce forged DNS information into the cache of a caching nameserver. DNS cache poisoning is not a new concept; in fact, there are published articles that describe a number of inherent deficiencies in the DNS protocol and defects in common DNS implementations that facilitate DNS cache poisoning. The following are examples of these deficiencies and defects:
Insufficient transaction ID space—The DNS protocol specification includes a transaction ID field of 16 bits. If the specification is correctly implemented and the transaction ID is randomly selected with a strong random number generator, an attacker will require, on average, 32,768 attempts to successfully predict the ID. Some flawed implementations may use a smaller number of bits for this transaction ID, meaning that fewer attempts will be needed. Furthermore, there are known errors with the randomness of transaction IDs that are generated by a number of implementations. Amit Klein researched several affected implementations in 2007. These are known to those skilled in the art.
Multiple Outstanding Requests
Some implementations of DNS services contain a vulnerability in which multiple identical queries for the same resource record (RR) will generate multiple outstanding queries for that RR. This condition leads to the feasibility of a ‘birthday attack,’ which significantly raises an attacker's chance of success. A number of vendors and implementations have already added mitigations to address this issue.
Fixed Source Port for Generating Queries
Some current implementations allocate an arbitrary port at startup (sometimes selected at random) and reuse this source port for all outgoing queries. In some implementations, the source port for outgoing queries is fixed at the traditional assigned DNS server port number, 53/udp.
Recent additional research into these issues and methods of combining them to conduct improved cache poisoning attacks have yielded extremely effective exploitation techniques. Caching DNS resolvers are primarily at risk—both those that are open (a DNS resolver is open if it provides recursive name resolution for clients outside of its administrative domain), and those that are not. These caching resolvers are the most common target for attackers; however, stub resolvers are also at risk.
Because attacks against these vulnerabilities all rely on an attacker's ability to predictably spoof traffic, the implementation of per-query source port randomization in the server presents a practical mitigation against these attacks within the boundaries of the current protocol specification. Randomized source ports can be used to gain approximately 16 additional bits of randomness in the data that an attacker must guess. Although there are technically 65,535 ports, implementers cannot allocate all of them (port numbers<1024 may be reserved, other ports may already be allocated, etc.). However, randomizing the ports that are available adds a significant amount of attack resiliency. It is important to note that without changes to the DNS protocol, such as those that the DNS Security Extensions (DNSSEC) introduce, these mitigations cannot completely prevent cache poisoning. However, if properly implemented, the mitigations reduce an attacker's chances of success by several orders of magnitude and make attacks impractical.
II. Impact —An attacker with the ability to conduct a successful cache poisoning attack can cause a nameserver's clients to contact the incorrect, and possibly malicious, hosts for particular services. Consequently, web traffic, email, and other important network data can be redirected to systems under the attacker's control.
Once a DNS server has received non-authentic data and caches it for future performance increase, it is considered poisoned, supplying the non-authentic data to the clients of the server until it expires the data.
Normally, an Internet-connected computer uses a DNS server provided by the computer owner's Internet Service Provider, or ISP. This DNS server generally serves the ISP's own customers only and contains a small amount of DNS information cached by previous users of the server. A poisoning attack on a single ISP DNS server can affect the users serviced directly by the compromised server or indirectly by its downstream server(s) if applicable.
To perform a cache poisoning attack, the attacker exploits a flaw in the Domain Name System architecture which allows it to accept incorrect information. If the server does not correctly validate DNS responses to ensure that they have come from an authoritative source, the server will end up caching the incorrect entries locally and serve them to users that make the same request.
This technique can be used to replace arbitrary content for a set of victims with content of an attacker's choosing. For example, an attacker poisons the IP address DNS entries for a target website on a given DNS server, replacing them with the IP address of a server he controls. He then creates fake entries for files on the server they control with names matching those on the target server. These files could contain malicious content, such as a worm or a virus. A user whose computer has referenced the poisoned DNS server would be tricked into thinking that the content comes from the target server and unknowingly download malicious content.
An early simple variant of DNS of cache poisoning involved redirecting the nameserver of the attacker's domain to the nameserver of the target domain, then assigning that nameserver an IP address specified by the attacker. A vulnerable server would cache an additional A-record (IP address) provided in response allowing the attacker to resolve queries to the domain provided as an additional A-record.
A second later variant of DNS cache poisoning involves redirecting the nameserver of another domain unrelated to the original request to an IP address specified by the attacker. A vulnerable server would cache the unrelated authority information allowing the attacker to resolve queries to the unrelated domain.
The third and most serious variant of DNS cache poisoning, which is called DNS Forgery, involves beating the real answer to a recursive DNS query back to the DNS server. DNS requests contain a 16-bit transaction id, used to identify the response associated with a given request. If the attacker can successfully predict the value of the transaction id and return a reply first, the server will accept the attacker's response as valid. If the server randomizes the source port of the request, the attack may become more difficult, as the fake response must be sent to the same port that the request originated from.
By sending a number of simultaneous DNS requests to the server to force it to send more recursive requests, the probability of successfully predicting one of the request transaction ids increases to a high level.
The problems are inherent in the DNS protocol and its usage of UDP transfers. A proposed solution DNSSEC has been resistant to adoption because of the lack of critical mass. There is no immediate payback for adopting DNSSEC to the early adopters. A most serious attack is hijacking authority records. An attack starts with a flurry of queries, each for a different random name under the main domain. The first request causes the nameserver to perform the usual root-first resolution, but it eventually caches the valid values. Subsequent queries within this domain go directly to that nameserver, skipping the root steps.
But a request for a different random name intentionally chosen to be not found in cache causes an immediate query to the valid ns1 server. The attacker then massively floods forged data at the victim about that second random name. Although no one forged reply has a high probability of success, a tiny percentage of a large number is non-zero.
By sending many forged replies for each random name query before the real reply arrives from the real nameserver the attacker has good chance of success at little cost.
By poisoning the Authority records for .COM and the like, the victim nameserver will route all DNS lookups to the attacker's nameservers. This effectively hijacks all names under that top level.
DNS poisoning and other methods of mis-representing DNS could lead to an unsuspecting person thinking he is at a website that is different than the website he is really viewing. This could lead the person to enter confidential passwords, account, credentials or other information. It would be difficult if not impossible for a user to detect this attack. There are many products that sit in the path of the DNS traffic and route the data for the DNS request and the DNS response. These products include Ethernet switches, IPS devices, Routers, web filters, and many others. A problem with solutions to address the vulnerability in the domain name system is that they can be undone or substantially weakened by routers, firewalls, proxies, and other gateway devices that perform Network Address Translation (NAT)—more specifically Port Address Translation (PAT)—which often rewrite source ports in order to track connection state. When modifying source ports, PAT devices can reduce source port randomness implemented by nameservers and stub resolvers (conversely a PAT device can also increase randomness). A PAT device can reduce or eliminate improvements gained by patching DNS software to implement source port randomization. Thus it can be appreciated that what is needed is a way to automatically verify DNS accuracy by independent means.